Nitride compound semiconductor materials are compound semiconductor materials which contain nitrogen, such as the above-mentioned materials selected from the system InxAlyGa1-x-yN, where 0≦x≦1, 0≦y≦1 and x+y≦1. In the present instance, the group of radiation-emitting and/or radiation-detecting semiconductor chips based on nitride compound semiconductor material include in particular semiconductor chips in which the epitaxially produced semiconductor layer, which generally includes a layer sequence comprising different individual layers, includes at least one individual layer which contains a material from the nitride compound semiconductor material system. The semiconductor layer may, for example, include a conventional pn junction, a double heterostructure, a single quantum well structure (SQW structure) or a multiple quantum well structure (MQW structure). Structures of this type are known to the person skilled in the art and are therefore not explained in more detail at the present juncture. Examples of MQW structures of this type are described in the documents WO 01/39282, WO 98/31055, U.S. Pat. No. 5,831,277, EP 1 017 113 and U.S. Pat. No. 5,684,309, the content of disclosure of each of which in this respect is hereby incorporated by reference.
It is known for a semiconductor material to be grown epitaxially on a substrate whose lattice constant is matched to the lattice constant of the semiconductor material in order to obtain an improved crystal quality and pure crystal defects. Hitherto, a lattice-matched substrate for the nitride compound semiconductor materials which is also sufficiently suitable for the mass production of semiconductor chips of this type has not been disclosed. Therefore, substrates based on sapphire, silicon carbide or spinel are frequently used, even though their lattice constant is not optimally matched to that of nitride compound semiconductor material.
Since it is intended to use the nitride compound semiconductors to fabricate optoelectronic components, in particular semiconductor lasers, and since these components may give off heat that is produced because of high electrical power losses of the components, the material sapphire is of only extremely limited suitability for the fabrication of power laser diodes, on account of its poor thermal conductivity.
It is also known to use special deposition processes to reduce the defect density in the semiconductor material. An example of a process of this nature for lateral overgrowth, which is often referred to as the LEO (lateral epitaxial overgrowth) process or the ELOG (epitaxial lateral overgrowth) process, is known from Song et al., Phys. Stat. Sol. (a) 180, 247 (2000), the content of which in this respect is hereby incorporated by reference.
In the process which is described therein for the production of a gallium nitride layer on a sapphire substrate, first of all a thin initial layer (seed layer) is grown on a sapphire substrate, and then a silicon nitride mask layer in strip form is applied to the thin initial layer. During subsequent deposition of trimethylgallium and ammonia, a plurality of gallium nitride layers grow between the mask strips. As soon as the gallium nitride layers have reached the thickness of the mask layer, lateral growth occurs in addition to the vertical growth, so that the mask layer is laterally overgrown by the gallium nitride layers. This process is continued until a continuous gallium nitride layer is present.
It has been found that the dislocation density in the gallium nitride layer produced by lateral overgrowth is advantageously low and is distinguished by a higher crystal quality in particular compared to a layer which is grown directly on the sapphire substrate.